In many high temperature applications, there is a requirement for valves which will seal and control the flow of high temperature exhaust gases. It is, however, extremely difficult in this high temperature environment, which in many situations is also a high temperature contaminated environment, to provide flow control valves without using very expensive materials or exotic designs. Most commonly used materials for valves for low temperature operations cannot stand up to high temperature or contaminated gas streams with the expectation that such valves will have a useful economical life expectancy.
An example of such environment is in a steel mill, where the off-gases from the blast furnace are hot, dirty and combustible. These off-gases are typically routed through a "stove" where the stove bricks are heated by the gases. While the stove is being heated, a second stove, previously heated by the off-gases, preheats combustion air flowing through it for combustion in the blast furnace. Combustion air is therefore routed alternately between two or more such "stoves" in order to preheat the combustion air. Where two such stoves are used, at least two three-way valves are employed to route the exhaust gas from the blast furnace to one or the other of the stoves and the preheated combustion air from the stoves to the blast furnace, respectively. These valves are necessarily specifically designed and use exotic materials to withstand the high temperatures to which they are exposed in order to control the flow of the high temperature gases in the various conduits.
Similarly as in the above-described steel mill application, there are a number of regenerative fume incinerators that use the afterburner off-gases to alternately heat refractory media such that the refractory media can alternately preheat gases, usually contaminated, to be incinerated. Valving arrangements, as previously described in the steel mill application, are likewise necessary in regenerative fume incinerators.
Another application of a valve used in high temperature applications is in hazardous waste incinerators. In equipment of this type, there is typically an emergency exhaust vent for dumping the gases in the event of the failure of a quench system. Because of the high temperatures and the need to use refractories, the vent becomes very heavy. This, in turn, presents problems in opening the valve in the event of power failure, as well as obtaining a good seal on the valve to avoid leakage during normal operation.
A further example of a valve for use in high temperature operations is in cogeneration plants. To produce maximum electrical generation from a turbine, oftentimes the exhaust gases are bypassed around the waste heat boiler. Those exhaust gases are at sufficiently high temperature to require valves of exotic designs and specialized, oftentimes expensive, materials. The high temperature flow control apparatus of the present invention, as indicated above, has many applications in a high temperature gaseous environment. The following description relates to the use of a high temperature valve for use in the exhaust gas conduit of a soil remediation unit. For controlling the flow of high temperature exhaust gases from a burner and particularly for controlling the temperature of such exhaust gases used to preheat contaminated gases flowing to an afterburner, it will be appreciated that the invention has the more general application described above.
While the preferred embodiment of the present invention is described herein in relation to the remediation of soil contaminated with hydrocarbons, it will be appreciated that materials other than soil may be remediated in the practice of this invention. For example, the present invention may be used to eliminate volatile organics from metal turnings, sludges, drilling muds, inorganic chemicals, etc., with the temperature of the heat exchanger being controlled in a thermally efficient manner.
Soils are frequently contaminated with volatile organics, e.g., hydrocarbon products, and this constitutes a highly significant and major pollution problem. The contaminants may range from gasoline through heavy hydrocarbon products and hydrocarbon chemicals, such as PCBs. Various efforts have been directed to remediating contaminated material, such as soil, and one of the most effective is to thermally treat the material. High cost, however, is an inhibiting factor and, in many cases, is the result of inefficiently designed equipment and limited equipment capabilities. For example, a major factor affecting cost is fuel efficiencies in the downstream treatment of the residual gaseous components driven off from the contaminated materials. In prior systems, the basic process for cleaning contaminated material such as soil is to expose it to high temperatures, where the contaminants are volatilized and subsequently burned.
In a conventional soil remediation unit, a rotary drum is provided having a contaminated soil inlet and a clean soil outlet at respective opposite ends of the drum. A burner flows hot gases of combustion in counterflow relation to the contaminated soil passing through the drum. The flow of hot combustion gases, in contact with the contaminated soil, volatilizes the contaminants and substantially remediates the soil. The hot gases of combustion containing the volatilized contaminants, as well as particulates from the soil, such as dust, are passed through a separator, where the particles are separated out. The resulting exhaust gases containing the volatilized contaminants are then passed into an afterburner, where the contaminants are completely burned to provide a clean exhaust gas. The volatilized contaminants thus provide fuel for the afterburner. In thermally efficient systems of this type, the high temperature stack gases from the afterburner are disposed in heat exchange relation with the volatilized contaminant-laden exhaust gases incoming from the separator before the latter gases are disposed in the afterburner. Those volatilized, contaminant-laden exhaust gases are therefore preheated, thereby conserving the fuel supply to the afterburner. The heat exchanger also cools the stack gases. Consequently, the heat exchanger adds substantial efficiency to the system.
It has been found, however, that for high contamination levels in the material, such as soils, more fuel than actually needed in the afterburner is supplied to the afterburner in the form of the volatilized contaminants, resulting in a runaway condition with respect to the temperature of the gases exhausting from the afterburner. Stated differently, if the contaminated soil has one or more "sweet spots," and sufficient organic material is driven off in the drum to supply more fuel in the afterburner than the afterburner needs, the exhaust gas temperature of the afterburner will rise as those additional fuel-supplying contaminants are burned. Thus, for low and moderate contamination levels in the material, i.e., soil, the heat exchanger serves to conserve the fuel supplied to the afterburner. For highly contaminated soil or sweet spots, the temperature of the exhaust gases rises to such an extent that it causes problems in the heat exchanger absent use of very expensive materials and constructions. It is therefore highly desirable to maintain the exhaust gas temperature of the afterburner below a predetermined maximum temperature, e.g., 1000.degree. F.
Bypassing the hot exhaust gases from the heat exchanger has been considered. However, bypassing these hot gases, which can reach as high as 1800.degree. F., constitutes a materials problem. For example, providing for thermal expansion in the heat exchanger, a valve which can seal off or modulate the exhaust gases when you want to bypass and still give reasonable life expectancy, and bearings for a damper assembly under those high temperature conditions causes very real practical problems requiring very expensive parts. Consequently, on the one hand, for low and moderately contaminated materials, use of a heat exchanger in preheating the volatilized contaminant-laden exhaust gases from the drum increases the efficiency of the afterburner. However, on the other hand, when the volatilized contaminants from highly contaminated materials or sweet spots are supplied and serve as fuel to the afterburner in excess of its capacity and the temperature therefore rises beyond acceptable limits, a runaway condition occurs in which the system, and particularly the heat exchanger, may be damaged by the high temperatures. Thus, an improved system is required which maximizes fuel efficiency for low and moderately contaminated soils with substantial fuel savings benefits, and enables bypass with modulation of very hot exhaust gases when highly contaminated soils are encountered.